Crash on latest version

[cblage]

When using the latest version there's an error happening:

Run eval "`devbox shellenv --init-hook`"
/home/runner/work/_temp/1b7f26cb-d5f8-4cd0-be72-3fc3a797f4e9.sh: line 199: export: `nix-version=2.25.3': not a valid identifier

It's related to this line in action.yml

echo "nix-version=$NIX_VERSION" >> $GITHUB_ENV

It seems like it's trying to create an ENV_VAR with hyphens - which is not possible.

Thanks!